1. Field of the Invention
The present invention relates generally to DC-DC converters and in particular to converters capable of operating in a step-down mode (buck) and in a step-up mode (boost) and smoothly transitioning between those modes.
2. Description of Related Art
DC-DC converters using various switching modes are commonly used to convert a DC input voltage to a regulated DC output voltage for powering a load. In a typical application input voltage Vin is provided by battery power source which has a relatively high output voltage when freshly charged, with that output voltage dropping as the battery becomes discharged. By way of example, assume that the charged battery voltage is initially +8 volts, with that voltage dropping to +5 volts as the battery becomes discharged and assume that the regulated output voltage supplied to a load is at +6 volts. Initially the converter needs to step down the voltage from +8 volts to +5 volts thereby requiring buck mode operation. Once the battery has discharged to +4 volts, a boost operating mode is required. When the battery voltage is near the output voltage of +5 volts, some sort of transition operating mode would be beneficial.
FIG. 1 is a diagram of an exemplary prior art DC-DC switching circuit 20 capable of both boost and buck operation. The control circuitry and output filter capacitor are not depicted. An inductor L1 is provided for storing energy received from the power source Vin connected to an input terminal 20A during a charge phase, with that energy then being transferred to the load at Vout connected to output terminal 20B during a discharge phase.
For typical buck operation, P type transistor switch M1 and N type transistor switch M2 are switched ON and OFF whereas P type transistor switch M3 remains constantly ON and N type transistor switch M4 remains constantly OFF. During the charge phase M1 is turned ON and M2 is turned OFF, with M2 being turned OFF first so that Vin is not temporarily shorted to ground. The voltage across the inductor is fixed at about VL=Vin−Vout, with the current increasing somewhat linearly and having a slope proportional to VL. When the inductor current reaches some peak value, transistor M1 is turned OFF and M2 is then turned ON thereby ending the charge phase and starting the discharge phase. The voltage VL across inductor L is again fixed at VL=Vout−0 with the direction of the current reversing so that the inductor L discharging into the filter capacitor (not depicted) and the load (not depicted). Regulator operation controls the duty cycle D which determines the output voltage Vout magnitude as follows for buck operation:Vout=D(Vin)  Eq. (1)                where D is the duty cycle of switch M1 M1ot/(M1ot+M2ot) with M1ot being the ON time for switch M1 and M2ot being the ON time for switch M2, with the sum being the duration of the switching period Ts.        (Equation (1) assumes continuous conduction operation where inductor L is always conducting current in one direction or the other.)        
As can be seen from Eq. (1), since D can never exceed one, the largest theoretical output voltage Vout for buck operation is Vin.
Note that transistor M2 acts as a rectifier and could be replaced with a diode. (Diode D1 is a body diode associated with M2 and not a separate diode.) There are advantages to replacing a diode with a transistor and switching the transistor ON and OFF as required, with this approach being referred to as synchronous rectification.
For typical boost operation, P type transistor switch M1 remains constantly ON and N type transistor switch M2 remains constantly OFF. During the charge phase, transistor M4 is ON so that the voltage VL across the inductor is VL=Vin−0. Thus the current though inductor L increases linearly at a rate proportional to Vin. Once the current through inductor L has increased to a peak value, M4 is turned OFF and M3 is turned ON so that inductor L discharge current will flow to the filter capacitor and load.
Once again, the duty cycle D is controlled to provide the desired regulated output voltage Vout in accordance with the following equation:Vout=(Vin)/(1−D)  Eq. (2)                where D is defined above in connection with Eq. (1).        (Equation (2) also assumes continuous conduction operation where inductor L is always conducting current in one direction or the other.)        
As can be seen from Eq. (2), since D can never exceed one, the smallest theoretical output voltage Vout for boost operation is Vin.
Also, transistor M3 could be replaced with a diode but synchronous operation using a transistor is usually preferred.
Converter operation in the region between the buck and boost operating modes where Vin and Vout are nearly equal to one another can become problematic. There is a need for a DC-DC converter capable of both buck and boost mode operation and further capable of a smooth transition between these operating modes.